The present invention relates to two-phase mixtures incorporating a liquid phase in which flows a gaseous phase in the form of bubbles, such as is encountered e.g. in bubble columns or chemical reactors.
More specifically, it relates to the determination, in such a medium, of the value of the interfacial area, i.e. the contact surface between the gas bubbles and the liquid phase per unit of volume. Thus, the interfacial area of such a medium is a basic parameter governing the mass of heat exchanges in liquid-gas contactors of the type conventionally used in chemical engineering. Therefore, the measurement of this quantity is of vital importance for studying and checking the operation of such contactors, as well as their development.
It is pointed out that the interfacial area per volume unit, also called the specific surface, is given by the relation: EQU .gamma.=(6.alpha.)/Deq
in which .alpha. is the vacuum level equal to the ratio of the gas volume to the total volume of gas plus liquid within the flow and Deq is the equivalent diameter of the gas bubbles present in the flow.
As is known, the vacuum level .alpha. can e.g. be measured by a manometric method, whilst the equivalent diameter of the bubbles is calculated by a photographic method.
Hitherto, there are four methods for measuring the interfacial areas of such a two-phase mixture flowing in a pipe or a stirred reactor.
The first method is a photographic method consisting of measuring the diameter of the bubbles after making a photographic negative thereof. It suffers from the disadvantage of requiring the two-phase mixture to be in a transparent container, of only permitting measurements to be carried out on groups of bubbles located close to the wall and particularly of requiring a long and tedious analysis, during which each bubble must be individually measured.
The second known method is a chemical method consisting of bringing about oxidation of an aqueous sodium sulphite solution by the oxygen contained in the gaseous phase. This method has the disadvantage of being limited to permanent flows and of requiring the use of special complementary fluids differing from those directly participating in the industrial mass or heat exchange process. Moreover, the results obtained are dependent on the geometrical shape of the contactor. In this connection, it requires a precalibration on a reference contactor, whose development and operation are far from easy. Finally, the practical performance of this chemical method is of long duration and requires several days for obtaining a correct estimate of the measured interfacial area.
The third method is an optical method using the attenuation of the intensity of a light beam passing through the container in which the two-phase mixture is flowing. This optical method has the obvious disadvantage of requiring the container to have transparent walls.
The fourth known method uses local interface detection probes, which are introduced into the gas-liquid contactor. Therefore, it suffers from the serious disadvantage of disturbing the flow which it is wished to examine, whilst also making it necessary to provide passages for the introduction of probes in the container walls or the pipe containing the two-phase flow. Finally, this method can only be used in the case of bubble columns.